MCGILL (CAN) — An environmental trifecta of old-growth trees, the moss that grows on them, and the nutrients contributed by cyanobacteria, work together to ensure a forest’s long-term survival.

Maintaining large ancient trees in coastal temperate rainforests that stretch from Southern Alaska to Northern California is important because, according to a new study, it is the interactions between old trees, mosses, and cyanobacteria, that contribute to nutrient dynamics that in turn sustain forests’ long-term productivity.

Bacteria that live in mosses on tree branches are twice as effective at fixing nitrogen as moss that grows on the ground, but moss doesn’t start to grow on many trees until they are more than 100 years old.

“What we’re doing is putting large old trees into a context where they’re an integral part of what a forest is,” says Zoe Lindo, postdoctoral fellow of biology at McGill University.

“These large old trees are doing something: they’re providing habitat for something that provides habitat for something else that’s fertilizing the forest. It’s like a domino effect; it’s indirect but without the first step, without the trees, none of it could happen.”

There are three players in the story, Lindo says: Large, old trees, mosses that grow along their branches, and cyanobacteria associated with the mosses.

The cyanobacteria take nitrogen from the atmosphere and make it available to plants—a process called “nitrogen fixation” that very few organisms can do. Forests’ growth and development is thought to be limited by the availability of nitrogen.

Cyanobacteria in mosses on the ground were recently shown to supply nitrogen to the Boreal forest, but until now cyanobacteria have not been studied in coastal forests or in canopies (tree-tops).

By collecting mosses on the forest floor and then at 15 and 30 meters up into the forest canopy, Lindo was able to show both that the cyanobacteria are more abundant in mosses high above the ground, and that they “fix” twice as much nitrogen as those associated with mosses on the forest floor.

Moss is the crucial element. The amount of nitrogen coming from the canopy depends on trees having mosses.

“You need trees that are large enough and old enough to start accumulating mosses before you can have the cyanobacteria that are associated with the mosses,” Lindo says.

“Many trees don’t start to accumulate mosses until they’re more than 100 years old. So it’s really the density of very large old trees that are draped in moss that is important at a forest stand level. We surveyed trees that are estimated as being between 500 and 800 years old.”

The research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC).